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stellar evolution - Coggle Diagram
stellar evolution
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main sequence
name of for a continuous and distinctive band of stars that appear on a plot of stellar colour vs brightness.
the more the massive the stars, the shorter its lifespan on the main sequence
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temperature gradient between the core of a star and its surface= energy steadily transported upward through the intervening layers until it is radiated away at the photosphere
convection
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when convection occurs in the core region it acts to stir up the helium ashes, thus maintaining the proportion of fuel needed for fusion to occur
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post-main sequence
a star remains on the main sequence as long as there is hydrogen in its core that it can fuse into helium
as main sequence star ages its luminosity increases slightly, resulting in it expanding and its outer layer cooling. this explains why the main sequence is a broad band rather than a narrow line -stars move up and to the right on this band as they age
expt: our sun
move off the main sequence and up the red giant branch - fusing hydrogen into helium in hydrogran shell burning
a very short helium flash sees the start of helium core fusion and the star moves along the horizontal giant branch
eventually hydrogen core runs out and fusion stops, shutting off the outward radiation pressure
inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy
rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star.
this so called shell-burning causes some interesting effects.
the new increased radiation pressure actually causes the puter layer of the star to expand to maintain the pressure gradient
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convection transport the energy to the outer layer of the stars from the shell burning region.
during this expansion, the star will move up and to the right on the HR diagram along the red giant branch. a G(V)- class star may end up as a high-K or low-M luminosity class 3 giant.
red giants
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mass loss from a giant is typically about 10^-7 solar masses per yrs, compared with only 10^-17 solar masses per yrs currently for the sun
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the horizontal branch
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a solar-mass star has sufficient helium fuel for the core-burning to last for abut 100 million years